The 3D structure of a protozoan hemoglobin may reveal whether or not the large sequence differences between the protozoan and higher animal hemoglobins translate into large structural differences. Data collected on hemoglobin yielded 2.3 E resolution data for the native and several derivative crystals. Heavy atom locations have been determined for two mercury derivatives. Crystals of rat pol b with template-primer and ddCTP decayed too quickly to collect a high resolution data set. Crystals will be supercooled to prevent decay and may reveal the important features of the nucleotidyl transfer reaction.